Transmit power efficiently
Step-up transformers raise potential difference for transmission; step-down transformers reduce it for consumers. Use the turns ratio and explain why high voltage reduces current and cable heating for a given power.
GCSE Physics resources
AQA GCSE Magnetism and electromagnetism
Transformers and the National Grid
Step-up transformers raise potential difference for transmission; step-down transformers reduce it for consumers. Use the turns ratio and explain why high voltage reduces current and cable heating for a given power.
Start here
The revision snapshot
Step-up transformers raise potential difference for transmission; step-down transformers reduce it for consumers. Use the turns ratio and explain why high voltage reduces current and cable heating for a given power.
By the end, you should be able to:
- Transmit power efficiently.
- Apply the magnetism and electromagnetism model, evidence or method to an unfamiliar exam context.
- Recognise and correct this wrong turn: Using d.c. as transformer input.
Before you revise
Diagnostic question
Choose an answer from memory. Your result tells you what to watch for in the guide.
Core revision guide
Learn the model, then use it
Transformers and the National Grid questions become manageable when the central model, evidence and exam method are kept together. Read the explanation, test the misconception and then apply the idea without looking back.
Build the physics picture
A transformer works with alternating current because the changing current in the primary coil creates a changing magnetic field. The turns ratio sets the voltage ratio. For a fixed transmitted power, higher voltage means lower current, so less energy is wasted heating the cables.
Relationships used in this guide
Transformer ratio: Vₚ ÷ Vₛ = Nₚ ÷ Nₛ (V, turns). Ideal transformer power: VₚIₚ = VₛIₛ (W). Choose each relationship from the physical change described, convert quantities into compatible units and keep the unit beside the final answer.
Misconception clinic
Replace the tempting answer
Read each belief, say what is wrong with it, then compare your correction.
Tempting idea: Using d.c. as transformer input.
Use instead: Step-up transformers raise potential difference for transmission; step-down transformers reduce it for consumers. Use the turns ratio and explain why high voltage reduces current and cable heating for a given power.
Tempting idea: Reversing primary/secondary ratio.
Use instead: A transformer works with alternating current because the changing current in the primary coil creates a changing magnetic field. The turns ratio sets the voltage ratio. For a fixed transmitted power, higher voltage means lower current, so less energy is wasted heating the cables.
Tempting idea: Saying high voltage itself “uses less energy” without mentioning current/heating.
Use instead: Identify primary/secondary, choose the ratio equation, calculate the missing value and finish with the transmission explanation.
Retrieval practice
Quick checks
Explore the relationship
Transformer turns-ratio model
Change primary voltage or the secondary:primary turns ratio, then use Vₛ = Vₚ × (Nₛ ÷ Nₚ).
What to notice: use the readout to describe how one variable changes when the other is controlled.
Worked examples
See the method being built
These examples expose the difference between a secure transmit power efficiently method and the common error “Using d.c. as transformer input.”.
Example 1 · Turns ratio
Primary: 200 turns, 230 V. Secondary: 1000 turns. Find secondary p.d.
- Use Vs/Vp = Ns/Np.
- Ratio = 1000/200 = 5.
- Multiply 230 by 5.
Show the answer
Vs = 1150 V.
Example 2 · Grid loss
Explain why the Grid transmits at high voltage.
- Use P = VI.
- For fixed power, high V means low I.
- Link low I to less heating loss.
Show the answer
Lower current reduces energy transferred to thermal stores of cables due to resistance.
Example 3 · AC requirement
Explain why a transformer does not work on steady d.c.
- Transformer needs changing magnetic field.
- Steady d.c. makes constant field.
- No induced secondary p.d.
Show the answer
A steady current does not continually change the magnetic field, so no continuous p.d. is induced in the secondary coil.
Exam precision
Exam technique: Transmit power efficiently
Do this: Identify primary/secondary, choose the ratio equation, calculate the missing value and finish with the transmission explanation.
Independent practice
Exam-style questions
Write an answer before opening the marking guidance. Then edit the exact phrase or step that would gain the next mark.
1. State why transformers need alternating current.
[1 mark]Show marking guidance and model answer
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: AC produces a changing magnetic field.
2. A transformer has twice as many secondary turns as primary turns. State voltage change.
[2 marks]Show marking guidance and model answer
Marking guidance: 2 marks are available for relevant, linked physics points that match the model answer.
Model answer: Secondary voltage is twice primary voltage for an ideal transformer.
3. State what a step-up transformer does.
[1 mark]Show marking guidance and model answer
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: Increases potential difference.
4. Explain why step-down transformers are used before homes.
[3 marks]Show marking guidance and model answer
Marking guidance: Award up to 3 marks for distinct physics points joined into a clear cause-and-effect chain.
Model answer: They reduce high transmission voltage to a safer/useful supply voltage.
5. State why high current in Grid cables is undesirable.
[1 mark]Show marking guidance and model answer
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: It causes more heating losses due to resistance.
6. State whether this route is Combined Science content.
[1 mark]Show marking guidance and model answer
Marking guidance: Award one mark for the precise statement shown in the model answer.
Model answer: No; it is AQA GCSE Physics Higher Tier content.
Questions pupils ask
Transformers and the National Grid FAQs
What is the main idea in Transformers and the National Grid?
Step-up transformers raise potential difference for transmission; step-down transformers reduce it for consumers. Use the turns ratio and explain why high voltage reduces current and cable heating for a given power.
What mistake should I avoid in Transformers and the National Grid?
Using d.c. as transformer input. A transformer works with alternating current because the changing current in the primary coil creates a changing magnetic field. The turns ratio sets the voltage ratio. For a fixed transmitted power, higher voltage means lower current, so less energy is wasted heating the cables.
Useful next steps
Connect this guide to your next task
Generator Effect and Electromagnetic InductionMoving a conductor through a magnetic field, or changing the magnetic field through a coil, induces a potential difference. Reversing the motion reverses the induced current direction.Magnetic Fields, Electromagnets and the Motor EffectA current produces a magnetic field, and a current-carrying conductor in an external field can experience a force. Use field diagrams and Fleming’s left-hand rule carefully for Higher demand.How to Answer GCSE Physics Calculation QuestionsA reliable calculation answer is a short chain of decisions. Make the physics visible so a marker can award method marks even if arithmetic slips.
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